In the field of powder drying, high demands on efficiency of the system are present in general, and a particular area of demand is the separation of dried products and process air/gas.
When drying powder products, a mixture of air/gas and dried powder particles exits a main powder drying processing unit, and enters a number of separation units. These units include cyclones and bag filter units, which can be combined in any number of ways. The function of cyclones and bag filters in a powder drying system is to separate the dried powder particles from the process gas/air. However there are limitations as to how high a percentage, of the dried powder particles, can be extracted from the mixture, at a given step. Cyclones, for example, can extract a fraction of especially larger particles, at a fairly low energy consumption, but are limited in regards to extracting the smaller particles. Bag filters are on the other hand effective at extracting even small particles from a mixed stream of air/gas and powder particles, as the particles will collect at the surface of the filters. However, bag filters have larger energy consumption, and also require more cleaning, as there is a larger number of components and a higher rate of filters being filled by dried powder particles.
In some fields, such as the pharmaceutical field, particularly concerning inhalable pharmaceutical substances, there is an increased demand on avoiding bag filters, as possible fibers from the bag filters may be detrimental to the final product, and should be avoided all together.
Further, in the field of powder drying systems, there is a high demand on reducing the overall size of the powder drying systems, in comparison to the dried powder product yield, as factory space is limited and costly.
There is an increasing demand on separation systems, which can increase the yield efficiency and/or reduce the energy consumption of the separation units. Some attempts at this have been made, such as the system used in WO 2015/075702 A1 or WO 2008/147233 A2, which uses a recirculation system. As usual, the cyclone separates a part of the mixed stream, by extracting a portion of dried powder at the powder outlet at the bottom of the cyclone. The remaining mixture of air/gas and dried powder particles exits the cyclone through the air outlet at the top of the cyclone. Upon exit of the cyclone, the mixed stream enters a swirl tube. Here a portion of the powder particles in the mixed stream agglomerate to larger particles, which are then extracted at the exit portion of the swirl tube and re-entered into the inlet stream of the cyclone. This, however, has the disadvantage of a high increase in energy consumption, as the pressure drop from inlet to outlet is much higher. Further, it has a disadvantage of being large, as the swirl tube is substantially the same size as the cyclone.